Selasa, 08 Oktober 2013

A sound approach to creating a quieter ride

Tina Jeffrey

Add sound to reduce noise levels inside the car. Yup, you read that right. And while it may seem counterintuitive, it’s precisely what automakers are doing to provide a better in-car experience. Let’s be clear: I’m not talking about playing a video of SpongeBob SquarePants on the rear-seat entertainment system to keep noisy kids quiet — although I can personally attest to the effectiveness of this method. Rather, I’m referring to deliberately synthesized sound played over a vehicle’s car speakers to cancel unwanted low-frequency engine tones in the passenger compartment, yielding a quieter and more pleasant ride.

So why is this even needed? It comes down to fuel economy. Automakers are continually looking at ways to reduce fuel consumption through techniques such as variable cylinder management (reducing the number of cylinders in operation under light engine load) and operating the engine at lower RPM. Some automakers are even cutting back on passive damping materials to decrease vehicle weight. These approaches do indeed reduce consumption, but they also result in more engine noise permeating the vehicle cabin, creating a noisier ride for occupants. To address the problem, noise vibration and harshness engineers (OEM engineers responsible for characterizing and improving sound quality in vehicles) are using innovative sound technologies such as active noise control (ANC).

Automotive ANC technology is analogous to the technology used in noise-cancelling headphones but is more difficult to implement, as developers must optimize the system based on the unique acoustic characteristics of the cabin interior. An ANC system must be able to function alongside a variety of other audio processing tasks such as audio playback, voice recognition, and hands-free communication.

The QNX Acoustics for Active Noise Control solution uses realtime engine data and sampled microphone data from the cabin to construct the “anti-noise” signal played over the car speakers.

So how does ANC work? According to the principle of superposition, sound waves will travel and reflect off glass, the dash, and other surfaces inside the car; interfere with each other; and yield a resultant wave of greater or lower amplitude to the original wave. The result varies according to where in the passenger compartment the signal is measured. At some locations, the waves will “add” (constructive interference); at other locations, the waves will “subtract” or cancel each other (destructive interference). Systems must be tuned and calibrated to ensure optimal performance at driver and passenger listening positions (aka “sweet spots”).

To reduce offending low-frequency engine tones (typically <150 Hz), an ANC system typically requires real-time engine data (including RPM) in addition to signals from the cabin microphones. The ANC system then synthesizes and emits “anti-noise” signals that are directly proportional but inverted to the original offending engine tones, via the car’s speakers. The net effect is a reduction of the offending tones.

According to the superposition principle of sound waves, a noise signal and an anti-noise signal will cancel each other if the signals are 180 degrees out of phase. Image adapted from Wikipedia.

Achieving optimal performance for these in-vehicle systems is complex, and here’s why. First off, there are multiple sources of sound inside a car — some desirable and some not. These include the infotainment system, conversation between vehicle occupants, the engine, road, wind, and structural vibrations from air intake valves or the exhaust. Also, every car interior has unique acoustic characteristics. The location and position of seats; the position, number, and type of speakers and microphones; and the materials used inside the cabin all play a role in how an ANC system performs.

To be truly effective, an ANC solution must adapt quickly to changes in vehicle cabin acoustics that result from changes in acceleration and deceleration, windows opening and closing, changes in passenger seat positions, and temperature changes. The solution must also be robust; it shouldn’t become unstable or degrade the audio quality inside the cabin should, for example, a microphone stop working.

The solution for every vehicle model must be calibrated and tuned to achieve optimal performance. Besides the vehicle model, engine noise characteristics, and number and arrangement of speakers and microphones, the embedded platform being used also plays a role when tuning the system. System tuning can, with conventional solutions, take months to reach optimal performance levels. Consequently, solutions that ease and accelerate the tuning process, and that integrate seamlessly into a customer’s application, are highly desirable.

Automotive ANC solutions — then and nowMost existing ANC systems for engine noise require a dedicated hardware control module. But automakers are beginning to realize that it’s more cost effective to integrate ANC into existing vehicle hardware systems, such as the infotainment head unit. This level of integration facilitates cooperation between different audio processing tasks, such as managing a hands-free call and reducing noise in the cabin.

Earlier today, QNX announced the availability of a brand new software product that targets ANC for engine tone reduction in passenger vehicles. It’s a flexible, software-based solution that can be ported to floating or fixed-point DSPs or application processors, including ARM, SHARC, and x86, and it supports systems with or without an OS. A host application that executes on the vehicle’s head unit or audio amplifier manages ANC through the library’s API calls. As a result, the host application can fully integrate ANC functionality with its other audio tasks and control the entire acoustic processing chain.

Eliminating BOM costsThe upshot is that the QNX ANC solution can match or supersede the performance of a dedicated hardware module — and we have the benchmarks to show it. Let me leave you with some of the highlights of the QNX Acoustics for Active Noise Control solution:

Significantly better performance than dedicated hardware solutions — The QNX solution can provide up to 9dB of reduction at the driver’s head position compared to 5dB for a comparative hardware solution in the same vehicle under the same conditions.

Flexible and configurable — Can be integrated into the application processor or DSP of an existing infotainment system or audio amplifier, and can run on systems with or without an OS, giving automakers implementation choices. Also supports up to 6 microphone and 6 speaker-channel configurations.

Faster time to market — Speeds development by shortening tuning efforts from many months to weeks. Also, a specialized team of QNX acoustic engineers can provide software support, consulting, calibration, and system tuning.